3
Electron Correlation In the Hartree-Fock approximation, each electron sees the average density (aka mean field) of all of the other electrons Two electrons cannot be in the same place at the same time Electrons must move two avoid each other, i.e. their motion is correlated Types of electron correlation –Dynamical –Non-dynamical The difference between the exact energy and the Hartree-Fock energy is the correlation energy for a particular basis set.

5
Motivation The wave function itself is essentially uninterpertable. Reduce problem size: Wave functions for N- electron systems contain 4N coordinates. Wave function based methods quickly become intractable for large systems, even with continued improvement in computing power, due to the coupled motion of the electrons. A desire to work with some physical observable rather than probability amplitude.

7
X  Model TFD does not predict bonding and the total energies are in error by %. If the  value in Slater’s E x is treated as parameter, then better results are achieved. The X  model (aka Hartree-Fock-Slater) uses  = 3/4. Although X  has been superceded by modern functionals, it is still useful for inorganic systems and preliminary calculations.

8
Hohenberg and Kohn (1964) –Energy is a functional of the density E[  ] –The functional is universal, independent of the system –The exact density minimizes E[  ] –Applies only to the ground state Kohn and Sham (1965) –Variational equations for a local functional –  can be written as a single Slater determinant of orbitals,but orbitals are not the same as Hartree-Fock –E XC takes care of electron correlation as well as exchange Theoretical Basis

9
Constructing Density Functionals Exact form is unknown. Hohenberg-Kohn is only an existence proof. Density functionals have the form: For LSDA: a = b = c = 0 For Pure Functionals: a = 0 Systematic improvement of functionals is possible, but complicated by the fact that exact constraints and properties of said functionals are still being elucidated.

14
Exchange-correlation functionals must be numerically integrated –not as robust as analytic methods Energies and gradients are 1-3 times the cost of Hartree-Fock Frequencies are 2-4 times the cost of HF Some of this computational cost can be recuperated for pure density functionals by employing the density fitting approximation for the Coulomb interaction. Calculating E xc Terms

15
DFT Performance For accurate energies some exact exchange is needed, although systematically improved functionals such as TPSS do outperform hybrids. GGA functionals offer a major improvement over LSDA in terms of energetics.The improvement from GGA to Meta-GGA is not as large as that from LSDA to GGA. Open-shell DFT calculations are less prone to spin contamination. Multiconfigurational problems and van der Waals interactions are poorly described.

16
DFT Literature Exchange-correlation hole density, n xc (r,r’) –describes how the presence of an electron at the point r depletes the total density of the other electrons at the point r’. Linear response function,  (r,r’;  ) –describes the change of total density a the point r due to a perturbing potential at the point r’ with a frequency . Energy density,  X or  C –energy density per particle

17
DFT Strategies As usual, start with minimal or DZ basis sets before attempting very diffuse and polarized ones. For preliminary calculations or to clean up starting structures, use LSDA rather than HF. This is particularly important for metal containing systems. Optimize structures with a pure functional to benefit from the density fitting speedup. Pay attention to the value and run stability checks when necessary. If a tighter integration grid is required, optimize with the default grid, then re-optimize with the tighter grid.